One of the more extraordinary things about the universe is that it has produced beings who can observe it — namely, us. Its laws and constants are so precise that, if they were even slightly modified, no human would be here to see it. Many cosmologists and philosophers have wondered if we should read anything into all this preciseness: Are the finely-tuned physical laws that surround us mere coincidence, or does it imply that we are somehow meant to be here? That's where the Anthropic Principle comes into play.

The Anthropic Principle (AP) is that hazy grey area where philosophy meets science. And in fact, many scientists loathe it for this very reason. It's untestable, they argue, and tautological — a skewed form of reasoning in which the principle is basically being used to prove itself.

And indeed, the AP does seem like a strange concept at first. It essentially states that we will only find ourselves in a universe that's capable of giving rise to us. Put another way, observations of the universe must be compatible with the conscious life that observes it.

It's a principle that makes perfect sense — and for some, no sense at all. But like so many things in science and philosophy, the devil is in the details.

The AP forces us to take a giant step back and evaluate the conditions of the universe in consideration of our presence within it. For scientists, it's a kind of ‘40 foot perspective' that can help illuminate — and even possibly explain — some of the more surprising aspects of cosmology. And at the very least, it serves as a constant reality check to remind us that we will always be subject to observational selectional effects; no matter where we go, we will always be there.

A good thought experiment in this regard comes from the Canadian philosopher John Leslie. In his book, Universes, he asks us to imagine a man facing a firing squad of fifty expert marksman. After aiming and firing, the executioners miss their mark.

Now, there are two ways in which we can evaluate this surprising outcome. We can either shrug our shoulders and point to the obvious, that they they simply missed. Or we can come up with some explanations as to why they all missed. This latter point is very much at the heart of anthropic reasoning.

Origins

The AP has been around for quite some time, though it only really took on its modern form in the last forty years.

Early efforts to come to grips with observational effects were expressed in Hume's Dialogues Concerning Natural Religion, and Kant's ideas about how our experience of the world is formulated by our sensory and intellectual faculties. Back in the 1920s, James Jeans observed that, "the physical conditions under which life is possible form only a tiny fraction of the range of physical conditions which prevail in the universe as a whole." Likewise, his contemporary, Arthur Eddington, speculated about "selective subjectivism," the idea that the laws of nature are indirectly imposed by the human mind, which in turn determines (and constrains) what we know about the universe.

More recently, some scientists have used it to explain the series of bizarre "large-number coincidences" in physics and cosmology. These are the surprisingly large order-of-magnitude connections that exist between (apparently) unrelated physical constants and cosmological parameters.

For example, the electromagnetic force is 39 orders of magnitude stronger than gravity. If it was any closer in strength, stars would have collapsed long before life could emerge. Or, the universe's vacuum energy density is about 120 orders of magnitude lower than some theoretical estimates, which, if any higher, would have blown the universe apart. And the neutron is heavier than the proton — but not so heavy that neutrons cannot be bound in nuclei where conservation of energy prevents the neutrons from decaying. Without neutrons, we wouldn't have the heavier elements needed for building complex life. There are many other examples, each one pointing to extreme specificity.

In 1961, Robert. H. Dickie used a prototypical version of the AP to explain away these coincidences, saying that physicists were reading too much into it. These large numbers, he argued, are a necessary coincidence (or prerequisite) for the presence of intelligent beings. If these parameters were not so, life would not have arisen. And in turn, we wouldn't be here to marvel at the ‘surprisingness' of these physical constants and laws.

Enter Brandon Carter

Then, in 1974, the philosopher Brandon Carter kindled the modern interpretation of these ideas, what he dubbed the Anthropic Principle. But rather than settle on just one perspective or definition, he said there were two different ways we can approach the issue.

Specifically, he proposed the Weak Anthropic Principle (WAP) and the Strong Anthropic Principle (SAP). Both approaches imply that these anthropic coincidences were not the result of chance, but were instead built directly into the structure of the universe.

Of the WAP he said:

We must be prepared to take into account the fact that our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers.

And of the SAP he said:

The universe (and hence the fundamental parameters on which it depends) must
be such as to admit the creation of observers within it at some stage.

Indeed, the SAP is a bit of a mind frak. Carter essentially argued that, if the SAP is true, the universe must to give rise to intelligent observers. The WAP, on the other hand, simply implies that the universe we observe must have the conditions to support intelligent life, but that life doesn't necessarily have to arise.

So, if the SAP is true, then the universe is indeed here for us.

Keep in mind that these are philosophical thought experiments, and not scientific statements per se. To a certain extent, philosophers are the conjurors of proto-scientific concepts — musings that should in turn be proven or disproven through the application of the scientific method.

Moreover, this doesn't imply or prove that God or some other Prime Mover exists, though many have taken it to that extreme. All the AP does in this regard is tell us that the laws of the universe should be understood through the context of the presence of observers.

Interestingly, Carter later regretted using the word ‘anthropic.' It has misled some into thinking that he was referring to Homo sapiens specifically (or that observers were limited to carbon based life). But his principle applies to any observer anywhere in the universe.

For example, a dolphin, which is a conscious being, can be considered an observer. Same goes for a self-aware robot on the other side of the universe. Or more conceptually, imagine a universe in which only evolving streams of information can exist. Eventually, a self-aware algorithm could emerge that's capable of assessing its surroundings. This would be an observer, too, but one far removed from our own experience.

Since Carter's original elucidation, the AP has literally been re-interpreted and re-defined hundreds of times. Other proposed names include "self-locating belief" and "indexical information" (not difficult to see why these didn't catch on). The "fine-tuning argument", however, has gained traction as a kind of substitute term, or correlated area of inquiry.

One of the more interesting re-evaluations of Carter's original idea comes from the mathematician John Barrow and physicist Frank Tipler. They devised a third principle, the Final Anthropic Principle, which states that intelligent information processing must come into evidence in the universe, and, once it comes into existence, it will never die out.

If this is true, not only is the universe here for us, but its configuration is such that we will become its permanent residents (in some form or another).

Welcome to the multiverse

As noted, many scientists hate the AP — and often with a passion. Critics contend that it's a product of cyclical thinking, and that's its self-evident — or that life should be simply be thought of as mere epiphenomenon (our presence in the universe is merely a side-effect, or coincidence).

Others, like physicist Lee Smolin, argue that the characteristics of the universe can be explained in other ways, such as his theory of cosmological natural selection. As Smolin told io9, "The Anthropic Principle is simply incapable of making a falsifiable prediction for any kind of testable experiment."

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At the same time, however, scientists like Sir Martin Rees have found it to be quite helpful, particularly when applying Carter's WAP to some modern interpretations of cosmology. In fact, some physicists, like Rees, use it when explaining (and reconciling) the multiverse theory.

According to this theory, our universe is not the only one, and also not the only kind. Given the possibility of a near infinite set of variable universes, there could be alternative universes out there with different constants and parameters. In some universes, gravity will be stronger, or the speed of light slower, and so on.

In the space of all possible universes, therefore, there will be a small subset of universes in which life can exist, and a larger subset in which life is impossible. Clearly, we find ourselves in one of the life-friendly universes. Other life-friendly universes with slightly different laws, or alternative modalities, may allow for other types of observers, but observers nonetheless; they too will be subject to the anthropic effect.

On the other hand, universes that are unfriendly to life can never be observed — but that doesn't mean they're not out there. It's just that nobody will be able to document such universes and record their unique characteristics. Unless, of course, as some interpretations of quantum physics suggests, universes can only exist in the presence of observers; no observer, no universe.

The inescapable observation selection effect

Critics and proponents aside, there's one last aspect to the AP that needs to be brought out — and that's its role as an observational principle.

Tautology or not, and regardless of whether multiverses exist, it highlights a fundamental problem or limitation that all scientists face when they're making any kind of proclamation about the nature of the cosmos — and that is, as observers, we will always be subject to observational selection effects.

Consequently, it serves as a kind of reality check, one that's somewhat akin to a soft interpretation of the Heisenberg Uncertainty Principle, or even Plato's Cave. It's the oppressive realization that everything we observe is being observed. And that in order for it be be observed by that something, the environment has to be conducive for that something to exist. We can only take measurements and formulate judgements in a modality in which that can happen.

As Oxford philosopher Nick Bostrom has said, "all observations require the existence of an appropriately positioned observer." Indeed, our data is not only filtered by the limitations of our instruments, "but also by the precondition that somebody be there to ‘have' the data yielded by the instruments (and to build the instruments in the first place)." The biases that occur due to these preconditions are what's referred to as observation selection effects.

So, in answer to the headline of this article — is this universe here just for us — the Anthropic Principle alone cannot provide the answer. But it does force us to take pause and acknowledge the efficacy of the suggestion. Whether science can now run with it and provide us with an answer is an open question.

In the meantime, take solace in the fact that you're a piece of the universe that's observing itself.